More and more radio apparatuses like mobile phones are used for various applications as radio communication technologies remarkably grow these days. A recent trend to be noted is a radio apparatus of multiple uses, i.e., ordinary voice and data communications plus receiving digital TV broadcasting or short-range radio links. Such a radio apparatus is required to have a broader frequency range than conventional ones, and this trend is pushed forward by developing ultra-wide-band (UWB) technologies.
Broadening a frequency range of a radio apparatus needs broadening a frequency range of each component, and especially broadening a frequency range of an antenna is required. Otherwise a radio apparatus of multiple uses is equipped with a plurality of conventional antennas of narrow frequency ranges (narrow-band antennas) and is greater in size and more complicated in its configuration than that of a single use. Such a radio apparatus having two or more narrow-band antennas is inconvenient to use or handle as a narrow-band antenna such as a whip antenna generally sticks out of a case of the radio apparatus.
Known is a conventional radio apparatus that uses a plane-shaped conductor, e.g., a printed pattern of a printed circuit board (PCB) contained therein as a built-in type antenna element. An example of such a conventional radio apparatus is disclosed in the following reference: Nishikido, T. et al, “An antenna system with two parallel elements and a dividing feed circuit for a handy phone”, IEICE Communications Society Conference B-1-190 (in Japanese), The Institute of Electronics, Information and Communication Engineers, September 2003.
Another example is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), 2004-56426, the English version of which is available on a website named “Intellectual Property Digital Library” linked from the Japan Patent Office website.
This conventional radio apparatus is a folding mobile phone called a clamshell type, and has an upper case containing an upper antenna element and a lower case containing a lower antenna element. Those antenna elements form a dipole antenna together, and its radiation characteristic is variably controlled with the shapes of the antenna elements and variations of inter-element connections.
This configuration, however, still has a disadvantage that the dipole antenna needs a balanced feed system that does not match an ordinary radio circuit having an unbalanced antenna connecting port and thus needs a converter. It has another disadvantage that the upper antenna element and a ground plane of the upper case are in plural layers, which makes a thinner case design difficult.
A mobile phone of a clamshell type is used for communications while its upper and lower cases are being closed as well as while they are being opened. An antenna and a feed system of such a mobile phone needs impedance matching whether the cases are being opened or closed. For the conventional configurations described above, it is difficult to meet such a need.